1. Field of the Invention
The present invention relates to an electronic ballast and is particularly concerned with such a ballast which comprises an inverter rectifier constructed in a switching bridge and to whose output side at least one load circuit composed of the series circuit of a lamp inductor with the parallel circuit composed of an ignition capacitor and a fluorescent lamp is connected. A regulator acting on the control of the switches of the first rectifier stabilizes the light current of the fluorescent lamp dependent on the lamp power or on the lamp current on the basis of a comparison between a reference value and a measured value derived from the lamp power or the lamp current and, simultaneously, enables a brightness regulation of the fluorescent lamp within broad limits dependent on the reference value which is a variable reference value.
2. Description of the Prior Art
Electronic ballasts of the type generally set forth above are disclosed, for example, by the German patent 37 09 004 A1. When such an electronic ballast is to be employed for dimming a fluorescent lamp within broad limits, particular difficulties arise given the settings &lt;10% of the nominal lighting current. Fluorescent lamps have great tolerances with respect to their electrical properties, sensitively react to temperature changes and are subjected to aging phenomena. When dimming the fluorescent lamp to low values, there is therefore the risk that the fluorescent lamp will go out because the discharge is interrupted.
In the aforementioned reference, the regulator regulates the brightness of the fluorescent lamp via its discharge current. This principle, however, fails given the settings &lt;10% of the nominal lighting current, since the differential current transformer needed for this purpose would have to be completely free of stray field. In the dimmed position of 1%, a stray field of the differential current transformer of only 1% of the main flow would falsify the measured result by approximately 100%.
As disclosed, for example, in the German patent 25 44 364 A1, the regulation can also occur via the lamp power instead of by way of regulating the discharge current of the fluorescent lamp. This, however, has the disadvantage that only the sum of lamp power and helices heating capacity can be regulated. The helices heating capacity is greatly dependent on the tolerance-affect helices resistance. This type of regulating can therefore only be conditionally employed given dimmed setting &lt;10% of the nominal light power. In a dimmed position of 1% of the nominal light power, for example, the light power to be regulated in standard fluorescent lamps amounts to about 0.5 W, but the heating capacity amounts to approximately 4 W. A satisfactory synchronism between a plurality of fluorescent lamps can thus not be guaranteed in this manner given the positions &lt;10%.